Magnetic-field and temperature dependence of the thermally activated dissipation in thin films of Bi2Sr2CaCu2O8+δ

The resistive transitions of a wide variety of Bi2Sr2CaCu2O8+δ thin films have been measured in μ0Hc^ from 0.01 to 15 T. For all samples, the transitions can be well approximated by the thermally activated form: R(T,H)Rnexp{-A(1-T/Tc)/kBT H } in the range 10-6Rn<R<10-2Rn. The energy scale A is 1740 K for smooth, in situ films made by molecular-beam epitaxy, and is 890 K for rough, polycrystalline films made by ambient temperature sputtering with an ex situ anneal. The field and temperature dependence of the activation energy, as well as its overall magnitude, is consistent with a model in which U(T,H) arises from plastic deformations of a viscous flux liquid above the vortex-glass transition temperature. The flux lattice shear mechanism proposed by this model is shown to be energetically preferable to direct lattice shear in highly anisotropic materials, thus explaining why the activation energy in Bi2Sr2CaCu2O 8+δ has a different field dependence than that for YBa2Cu3O7.